The invention relates to a powder metallurgy manufactured high speed steel with a high content of nitrogen in the form of a body formed through consolidation of alloyed metal powder. The invention particularly relates to a high speed steel suitable for cold work tools intended for applications where the tool is subjected to heavy friction between the working material and the tool resulting in a risk of adhesive wear.
Cold work often includes blanking, punching, deep drawing, and other forming of metallic working materials, which usually have the form of sheets or plates, normally at room temperature. For this type of work there are used cold work tools, on which a number of requirements are raised, which are difficult to combine. The tool material shall have a high resistance against abrasive wear, which among other things implies that it hall have an adequate hardness; it shall also have a good resistance against adhesive wear for certain applications; and it shall also have an adequate toughness in its use condition.
For the above and other applications there is to a great extent used a cold work steel which is known under its trade name Sverker 21(copyright), which is a conventionally manufactured steel having the composition 1.55 C, 0.3 Si, 03 Mn, 12.0 Cr, 0.8 Mo, 0.8 V, balance iron and impurities in normal amounts. For cold work tools there is also used the powder metallurgy manufactured tool steel which is known by its trade name Vanadis 4(copyright), which contains 1.5 C, 1.0 Si 0.4 Mn, 8.0 Cr, 1.5 Mo, 4.0 V, balance iron and impurities in normal amounts. Also high speed steels are employed, such as those high speed steels which are known under the trade names ASP(copyright)2023 and ASP(copyright)2053. The former one has the nominal composition 1.28 C, 4.2 Cr, 5.0 Mo, 6.4 W, 3.1 V, while the latter one has the nominal composition 2.45 C, 4.2 Cr, 3.1 Mo, 4.2 W, 8.0 V, wherein the balance in both the steels is iron, normal amount of Mn and Si and normally existing impurities.
The above mentioned and other steels available on the market place satisfy high requirements on abrasive wear resistance, toughness and other features. However, they do not satisfy high requirements on adhesive wear resistance, which often is a dominating problem in connection with different types of cold forming tool applications, such as pressing of sheets, pipe bending, and cold extrusion. These problems particularly can arise in connection with cold working of sheets of austenitic and ferritic stainless steels, copper, brass, aluminium, and others. The problems can be reduced by lubrication and/or surface coating of the tool surfaces with friction reducing ceramic layers of eg. TiN through PCD or CVD technique, through surface nitriding, or through hard chromium plating, but those are expensive and time consuming problem solutions. In addition, the risk of damages and/or flaking of the depositions is great. If abrasive or adhesive wear damages arise, the repair will be complicated because any defect always will be located on a very stressed part of the tool.
It is the purpose of the invention to provide a high speed steel for cold work tools with a very high resistance to adhesive wear in combination with other desirable fires of cold work tools, such as adequate toughness, harness, and resistance to abrasive wear. The steel shall, after pressing the powder to form a consolidated body through hot isostatic compaction (HIP-ing), be able to be hot worked through forging, rolling, and extrusion or be used in the as HIP-ed condition.
These and other objectives can be achieved therein that the high speed steel, with reference to its chemical composition in weight-%
1-2.5 C
1-3.5N
0.05-1.7 Mn
0.05-1.2 Si
3-6 Cr
2-5 Mo
0.5-5 W
6.2-17(V+2Nb)
balance iron and unavoidable impurities in normal amount, wherein the amount of, on one hand, the carbon equivalent, Ceq, expressed as       Ceq    =          C      +                        12          14                ⁢        N              ,
and, on the other hand, the vanadium equivalent, Veq, expressed as Ve=V+2 Nb, shall be balanced relative to each other such that the amounts of said elements, expressed in terms of said equivalent, will lie within the area A1-B1-C1-D1-A1 in the system of co-ordinates in FIG. 1, in which Ceq/Veq-co-ordinates of the point A1-D1 are
A1:4.5/17
B1:5.5/17
C1:2.5/6.2
D1 1.5/6.2
and that the high speed steel with reference to its structure, in the hardened and tempered condition of the steel, contain 12-40 vol-% of hard matter consisting of particles of MX-type, which are evenly distributed in the matrix of the steel, M in said matter of MX-type essentially consisting of vanadium and/or niobium, and X consisting of 30-50 weight-% carbon and 50-70 weight-% nitrogen.
In the following, a number of more limited areas will be defined, defining different embodiments and variants of the invention with reference to the relations between carbon equivalent and vanadium equivalent. I the list below, the Ceq/Veq-co-ordinates for all the points which have been indicated in the diagram in FIG. 1 have been stated.
In this text percentages refer to weight-%, unless otherwise is mentioned
A number of preferred or conceived embodiments of the invention with reference to the relation between the carbon and vanadium equivalents of the steel within the entire range Veq=6.2xe2x88x9217 (V+2 Nb) are stated in the sub-claims 2-5.
In the following, the election of the different alloy elements and the contents of them will be explained more in detail.
Carbon has two important functions in the steel of the invention. On one hand it shall, together with nitrogen and vanadium and/or niobium, form vanadium and/or niobium carbonitrides; on the other hand carbon shall exist in a sufficient amount in the matrix of the steel in order to provide a desired hardness of the martensite which is obtained after hardening and tempering. More particularly the content of carbon which is dissolved in the matrix should amount to 0.40-0.60%, preferably to 0.47-0.54. From these reasons, carbon shall exist in an amount of at least 1 weight-% and max 2.5 weight-%.
In said hard matter of MX-type, i.e. vanadium and/or niobium carbonitrides, X shall consist of 30-50 weight-% carbon and 50-70 weight-% nitrogen, wherein the ratio weight-% N/weight-% C of the amounts and nitrogen and carbon which are present in said carbonitrides of MX-type shall satisfy the conditions:   1.0  ≤            weight      ⁢              -            ⁢      %      ⁢      N              weight      ⁢              -            ⁢      %      ⁢      C        ≤      2.3    .  
The amount of nitrogen which exist in the steel in its molten state prior to gas granulation and the amount of nitrogen which is added to the steel by nitriding the gas granulated steel powder, which is the greater part, essentially combine with vanadium and/or niobium to form said carbonitrides. The amount of nitrogen which remains in the matrix of the steel and/or which possibly form nitrides with other existing elements, shall be practically negligible in comparison with the amount of nitrogen in said carbonitrides. For the achievement of the desired carbonitrides of MX-type, the content of nitrogen therefore shall amount to at least 1 weight-% and max 3.5 weight-%.
Silicon exists in an amount of at least 0.05, preferably at least 0.1% as a residual product from the deoxidation of the steel melt and can be tolerated in amounts up to 1.7%, preferably max 1.2%, normally max 0.7%.
Manganese exists in an amount of at least 0.05%, preferably at least 0.1%, in the first place as a residual product from the melt metallurgical process technique, where manganese is important in order to make sulphur compounds harmless through the formation of manganese sulphides in a manner known per se. The maximally tolerated manganese content is 1.7%, preferably max 1.0%, normally max 0.5%.
Chromium shall exist in the steel in an amount of at least 3%, preferably at least 3.5%, in order to contribute to the achievement of a sufficient hardenability of the matrix of the steel. Too much chromium, however, may cause a risk of retained austenite which is difficult to transform, and formation of M7C3-carbides, which are less desired. The chromium content therefore is limited to max 6%, preferably max 5%, and desirably max 4.5%.
Molybdenum and tungsten shall exist in the steel in order to provide a secondary hardening during tempering and to give a contribution to the hardenability. The limits are chosen such that the said elements, adapted to other alloy elements, shall provide an optimal hardness after hardening and tempering and also provide a small amount of hard M6C-particles. Molybdenum should exist in an amount of at least 2%, preferably at least 2.5% and suitably at least 3.0%. Tungsten should exist in an amount of at least 0.5% preferably in an amount of at least 2.0%, and suitably at least 2.5% and most conveniently at least 3.0%. The contents of each of molybdenum and tungsten, should not exceed 5% preferably not exceed 4.0%. As far as molybdenum and tungsten are concerned, the expression       Mo    eq    =      Mo    +          W      2      
should lie in the range 2.25-7.5%, preferably within the range 4-6%. The content of M6C-carbines, where M substantially consists of molybdenum and tungsten, should totally amount to 3.5 vol-% or to 10-30% of the total volume content of (MX+M6C)-phase.
Vanadium shall exist in the steel in a lowest amount of 6.2% and max 17% in order, together with carbon and nitrogen, to form very hard vanadium carbonitrides, i.e. hard matter of MX-type, where M essentially is vanadium and X is carbon and nitrogen in the weight ratios which have been mentioned in the foregoing. Possibly, vanadium may entirely or partially be replaced by niobium. The maximally allowed niobium content is 1.0%, preferably max 0.5%. Suitably, however, the steel does not contain any intentionally added niobium, because that can make the scrap handling in a steel work more complicated but above all because niobium might cause an impaired toughness of the steel because of a more unfavourable, more edgy carbide structure than a typical vanadium carbonitride of MX-type.
As has been mentioned in preamble, it is a purpose of the invention in the first place to provide a new high speed steel suited for cold work tools. Because cold work steels shall be able to be used at room temperature, the steel advantageously should not contain cobalt which is expensive and can make the steel less tough. According to a conceivable aspect of the invention, however, the steel should also be possible to be employed for working at high temperatures, in which case cobalt might be included in amounts up to max 20%, preferably max 12%. For the in the fist place intended field of usexe2x80x94cold work steelsxe2x80x94the steel, however, should not contain cobalt in amounts higher than those impurity contents which normally occur as residual elements from the raw material which are used in steel works which manufacture high speed steels, i.e. max 1% cobalt, preferably max 0.5% cobalt.
According to a first variant of the invention, the vanadium content shall be 6.2-9.5%. This implies, according to the widest aspect on this first variant, that the co-ordinates of the carbon and vanadium equivalents shall lie within the area G1-H1-C1-D1-G1 in the system of co-ordinates in FIG. 1.
Limiting aspects on this first variant are stated in the subsequent claims 7-12. Within the frame of the most limited aspect on this first variant is a steel having the following preferred, nominal composition: 1.3 C, 1.4 N, (Ceq about 2.5) 0.5 Si, 0.3 Mn, 4.2 Cr, 3.0 Mo, 4.0 W. 8.0 V, balance iron and normally existing impurities. Such a steel can be employed for most of the mentioned fields of use for which the steel is intended.
According to a second variant of the invention, the steel shall contain 13.5-17 (V+2 Nb). This implies, according to the widest aspect on this variant, that the co-ordinates of the carbon and vanadium equivalents shall lie within the area A1-B1-E1-F1-A1 in the system of co-ordinates in FIG. 1. Limiting aspects of this second variant are stated in the subsequent claims 14-19. Within the frame of this most limited, preferred composition according to this second aspect is a steel with the following preferred, nominal composition 2.0 C, 3.0 N, (Ceq about 4.6), 0.5 Si, 0.3 Mn, 4.2 Cr, 3.0 Mo, 4.0 W, 15.0 V, balance iron and normally existing impurities. A steel having this composition is particularly suited to be employed for the manufacturing of tools which are subjected to particularly heavy adhesive wear and differs from the foregoing preferred composition by it higher contents of vanadium, carbon, and nitrogen resulting in an about twice as high fraction of MX-phase.
According to a third variant of the invention, the steel shall contain 9.5-13.5 (V+2 Nb), wherein the coefficients of the contents of the carbon and vanadium equivalents lie within the area F1-E1-H1-G1-F1. Limiting aspects of this third variant are stated in the accompanying claims 21-26. Within the frame of this most limited, preferred composition according to this third variant there is a steel having the following preferred nominal composition: 1.5 C, 2.0 N (Ceq about 3.2), 0.5 Si 0.3 Mn, 4.2 Cr, 3.0 Mo, 4.0 W, 11.0 V, balance iron and normally existing impurities. A steel of that kind provides a better hot workability than the highly alloyed steel according to said second variant and also a better wear resistance than the less alloyed steel according to said first variant.
The technical features of the steel can be described as follows:
The steel consists of a powder metallurgy manufactured high speed steel, the alloy composition of which in the first place is distinguished by a high vanadium content. In its delivery condition the steel has a substantially ferritic matrix, which contains a considerable amount of carbonitrides, in the first place vanadium carbonitrides, which are fine-grained and evenly distributed in the steel.
After dissolution treatment in the temperature range 1000-1180xc2x0 C., preferably in the range 1050-1150xc2x0 C., and cooling to room temperature, the matrix of the steel has a predominantly martensitic structure but with a high content of retained austenite. Part of the carbonitrides and of the carbides which also exist in the steel, are dissolved, but 15-30 vol-% fine-grained, evenly distributed vanadium carbonitrides remain in the steel.
The hardness is increased to 58-66 HRC (the hardness within this range depends on the austenitising temperature) through tempering to a temperature within the temperature range 500-600xc2x0 C. because the retained austenite essentially has been eliminated and been transformed to martensite and by secondary precipitation of in the first place vanadium carbonitrides.
Because, in the first place, of the large content of vanadium carbonitrides, the hardened tempered steel is afforded a very high wear resistance at room temperature, and because of its combination of alloy elements, the steel in other respects is afforded a combination of hardness and toughness which is adequate for the type of cold work tools which has been mentioned in the preamble of this text.
The high speed steel of the invention can be manufactured in the following way. A melt is prepared in a conventional, melt metallurgical way, wherein the melt will get a nitrogen content which does not exceed the maximal content of nitrogen that can be dissolved in the molten steel, while the other alloying elements are adjusted to the contents which are stated in claim 1 or to any of the specified contents which are stated in the dependent claims. From this melt there is formed a metal powder, which can be carried out in a known way trough granulation of a steam of molten metal by means of gas-jets of nitrogen and/or of argon, i.e. according to the technique which forms an initial part of the so called ASProcess (Asea Stora Process). The powder is sieved to a suitable powder gauge, eg. max 250 xcexcm. Part of the powder is alloyed with nitrogen through solid phase nitriding by means of a nitrogen carrying gas, e.g. nitrogen and/or ammonia gas according to any technique which also may be known. Among known techniques which can be employed may be mentioned for example the technique which is disclosed in SE-C-462 837 or the technique which is described in MPR July, 1986 p. 527-530. Preferably there is used a gas mixture of ammonia and hydrogen gas which is caused to flow through a hot powder bed in a rotating reactor at 550-600xc2x0 C. The ammonia reacts at this temperature at the surface of the steel powder according to the reacton 2NH3xe2x86x923H2+2N (steel). Dissolved nitrogen then will diffuse from the surface into the powder grains. At the exit of the reactor the gas consists of a mixture of nitrogen, hydrogen, and a smaller amount of residual ammonia. The method allows a manufacturing of a nitrided material with a very accurate control of the content of nitrogen. A powder which is alloyed with nitrogen in this or in any other way is mixed with a powder which is not alloyed with nitrogen but which in other respects has preferably the same composition as the nitrogen alloyed powder, so that the mixture will get a desired mean nitrogen content according to the invention. This mixture is charged in sheet capsules which are closed and are hot isostatically compacted according to a known technique, preferably according to the technique which has been mentioned in the foregoing and which is known under the name ASP (Asea Stora Process), for the achievement of a consolidated body of a nitrogen alloyed high speed steel of the invention. This body can be hot worked through rolling and/or forging to desired dimension. During the process of consolidation and at the subsequent hot working, existing variations as far as the content of nitrogen in the starting material for the hot working are concerned, are levelled out so that all parts of the body will get an essentially equally high content of nitrogen.